The present invention relates generally to fluid operated tools, and more particularly to such fluid operated tools which have drive means and a ratchet-pawl mechanism including a ratchet connectable to a threaded connector to be tightened and loosened, for example a bolt, a nut, etc. and a pawl which is engageable with and disengageable from the teeth of the ratchet under the action of the drive means.
One of such tool is disclosed for example in U.S. Pat. No. 4,409,865. In this tool a lever mechanism has two drive pawls. While one drive pawl, engages the ratchet during a forward stroke of the piston, the other pawl is tilted back so as to slip over the ratchet teeth and to engage the ratchet tooth on the retract stroke of the piston while at that time the first drive pawl is tilted back. Therefore the ratchet turns forward both during the advance stroke and the retract stroke of the piston.
The problem is that during use of this tool on an actual bolting application, the nut is turned from tight to loose or from loose to tight. When the nut is being tightened and the turning resistance of the nut gets greater with each stroke, the flexing of the lever mechanism gets greater. This in turn requires a built-in overstroke which can be 50% more than the actually calculated stroke. This means, that on a loose nut the pawl which moves backward to catch another tooth falls in way behind that tooth and thus has a free forward movement when the piston moves the other way, until the front of the pawl hits the ratchet tooth. That unnecessary free forward movement can be quite extensive when the nut is loose and there is turning resistance, but becomes quite minimal when high torque is being applied to the nut. This means that when the nut is loose, a good part of the piston stroke is used just to overcome the overstroke and have the pawl engaged with the ratchet tooth. When the nut is tighter, on the other hand, the overstroke is absorbed by the material flex. Since the mechanism is described in the above mentioned patent allows for a continuous turning of the ratchet and thus the threaded connector which the ratchet engages, it is important to eliminate the overstroke when the nut is loose, since otherwise, the turning motion will be interrupted each time the tool switches from an advance stroke to a retract stroke.